Medical devices that can be advanced into the vasculature of a patient, and properly positioned at a site for an in-situ procedure, have several structural requirements in common with each other. Most importantly, they must be properly dimensioned to allow them to be advanced into the vasculature. This requires they be long and slender. Also, they must be steerable, bio-compatible, flexible and have sufficient structural strength to maintain their integrity while they are employed in the vasculature. With all of this in mind, the interventional device must also be fully capable of performing its intended function.
Recently, there has been substantial interest in medical procedures involving the cryo-ablation of tissue. In general, such procedures are intended to freeze specifically identified tissue. One procedure for which the cryoablation of tissue is known to be particularly efficacious is in the treatment of atrial fibrillation in the left atria of the heart. It happens, however, that cryoablation in general, and this procedure in particular, preferably requires temperatures below about minus eighty four degrees Centigrade (−84° C.). In order to generate such temperatures deep in the vasculature of a patient, several heat transfer principles need to be considered. Specifically, not only must such very low temperatures be generated, these temperatures must be somehow confined to the proximity where tissue is to be cryoablated.
Fourier's law of heat conduction states that the rate at which heat is transferred through a body, per unit cross sectional area, is proportional to the temperature gradient existing in the body (dQ/dt=rate of heat transfer). Mathematically, this phenomenon is expressed as:dQ/dt=−λAdT/dxwhere λ is the material's thermal conductivity, “A” is the cross sectional area through which heat is to be transferred, and dT/dx is the local temperature gradient. In the context of a cryo-catheter, “A” will be predetermined and will be necessarily limited by space considerations. Further, because high thermally conductive materials can be used in the manufacture of the cryo-catheter (e.g. copper), the thermal conductivity (λ) for a cryoablation procedure is effectively controlled by the relatively low conductivity of the tissue that is to be ablated. Thus, it can be appreciated that the local temperature gradient “dT/dx” is a control variable of significant importance. In particular, it is desirable that the local temperature gradient between tissue at an operational site, and the refrigerant in a cryo-catheter, be as great as possible. Stated differently, it is desirable to have cryo-catheter temperatures at the operational site that are as low as possible.
In addition to the temperature gradient effect discussed above, it is also to be appreciated that a substantial amount of heat transfer in a substance can result without any change in temperature. Specifically, this phenomenon involves latent heat and occurs wherever a substance, such as a fluid refrigerant, changes state. By definition, “latent heat” is the heat which is required to change the state of a unit mass of a substance from a solid to a liquid, or from a liquid to a gas, without a change of temperature. In the case of a fluid refrigerant, it can be said that prior to such a state change, the liquid refrigerant is “refrigerant in excess”. On the other hand, after the fluid refrigerant begins to boil (i.e. change state from liquid to gas) the gas refrigerant is “refrigerant limited”. Insofar as cryo-catheters are concerned, due to their requirement for low operational temperatures, it is desirable to obtain the additional refrigeration potential that results during the transfer of latent heat. Stated differently, it is preferable for the refrigerant to stay in its liquid state (i.e. remain “refrigerant in excess”) until employed for cryoablation.
At this point it should also be noted that there is a significant benefit which is obtained by maintaining a fluid refrigerant in its liquid state while it transits through a system. Specifically, this benefit comes from the fact that, any water entrained in the liquid refrigerant is prevented from forming as frost or ice that could clog the system, so long as the refrigerant remains liquid. This is a particularly important consideration whenever a system requires that the refrigerant pass through small or narrow orifices.
As discussed above, for the operation of a cryoablation system, it is necessary to select a fluid refrigerant that is capable of generating very low temperatures (i.e. <−84° C.). Prior to its use in the system, however, the fluid refrigerant is typically stored in vessels under very high pressure (i.e. around 700 psia). On the other hand, when it is to be used in a cryo-catheter, the pressure on the refrigerant needs to be reduced in stages to about one atmosphere. In addition to the refrigeration effect, an important consideration here is that the pressure be reduced to a level below normal blood pressure for safety reasons.
Although there are several well known ways in the pertinent art for reducing the pressure on a fluid, a convenient way for accomplishing this pressure reduction in a cryo-catheter is by passing the fluid refrigerant through a capillary tube. For capillary tubes that can be considered as being long, straight, uniform pipes, the “Darcy equation” is applicable. According to the Darcy equation a pressure drop along the length of the pipe (tube) (i.e. head loss “hl”) can be mathematically expressed as:hl=f(l/d)(V2/2g)
In the above expression: “f” is a friction factor, “l” is the length of the tube, “d” is the diameter of the tube, “V” is the velocity of the fluid through the tube, and “g” is the acceleration due to gravity.
From the Darcy equation it is to be noted that the head loss (hl) is proportional to the ratio “l/d”. This is the same as saying that the head loss is inversely proportional to the aspect ratio (“d/l”) of the pipe (tube). Regardless how viewed, the pressure drop along the entire length of a pipe will increase by reducing the inside diameter of the pipe “d” or by increasing the length “l” of the pipe. In any event, the dimensions of a tube that is to be used in a cryo-catheter for the purpose of reducing pressure on a fluid refrigerant should be selected so that the fluid is “refrigerant in excess” (i.e. in a liquid state) as it transits through the tube. Empirical results can be helpful when determining the most effective dimensions for such a tube.
In light of the above, it is an object of the present invention to provide a heat transfer system that will maintain a fluid refrigerant in a liquid state during a pressure drop on the fluid that is greater than four hundred psia, when the final pressure on the fluid is to be less than approximately one atmosphere. Another object of the present invention is to provide a heat transfer system that effectively avoids frost or ice build-up in the system as refrigerant passes through a relatively small orifice. Still another object of the present invention is to provide a heat transfer system that can be safely introduced into the vasculature of a patient where it will create temperatures as low as about minus eighty four degrees Centigrade. Another object of the present invention is to provide a heat transfer system that is relatively easy to manufacture, is simple to use and is comparatively cost effective.